Internal combustion engines are subject to knocking under certain operating conditions. Knocking, as usually understood, is defined as a spontaneous combustion, in the form of a pulse wave or shock wave within the cylinder, resulting in substantial pressure oscillations within the cylinder and high temperatures at the surfaces of the walls of the piston and cylinder facing the combustion chamber. The wall surfaces are thermally overloaded, which is dangerous and damaging thereto. Knocking, thus, should be avoided wherever possible. Knocking results in material removal from the wall surfaces, thus damaging the surfaces; this damaging material removal particularly occurs at the surfaces of the piston, the piston rings, the cylinder surface exposed to the combustion temperature, and the cylinder head seal, for example a cylinder head gasket. Upon increase of occurrence of knocking, and its intensity, the consequent material removal and other damage, including heat stresses, eventually leads to overall damage of the engine and may lead to its destruction.
It is desirable to operate internal combustion engines at the limit of their operating capability and efficiency. It is thus important to operate the combustion engine just below the knocking level, that is, with ignition timing and fuel supply such that the maximum combustion effectiveness of the chamber can be utilized without, however, resulting in premature or uncontrolled combustion which has knocking as a consequence. It is thus desirable to provide a sensing arrangement which early and reliably indicates any knocking phenomena before they can build up to destructive levels.
Various types of sensor arrangements have been proposed; some of those sensor arrangements which are known utilize occurrence of audio frequency oscillations as a sensing parameter, which occurs upon engine knocking. The audio frequency oscillations are mechanically sensed and converted into electrical signals, for example by using piezoelectrical pressure sensors located in a special bore in the cylinder head of the internal combustion engine, or attached to a component which transfers such oscillations. A sensing arrangement of this type is described, for example, in the referenced application Ser. No. 06/003,811, filed Jan. 16, 1979, GAST, now U.S. Pat. No. 4,275,586.
Sensors which respond to mechanical oscillations occurring within the cylinder head are capable of reliably indicating engine knocking. Since these sensors respond to mechanical oscillations, however, they will also respond to oscillations which are induced not only by knocking, but are derived from the operation of the vehicle as such, for example jolts or impacts on the vehicle due to road conditions. Thus, such sensors may indicate knocking when actually no knocking occurs since the sensor responded to a spurious oscillation. The parameter which is being measured, namely oscillation due to knocking, is transduced from a mechanical parameter into an electrical signal having an indicating parameter representative of knocking.